Please refer to FIG. 1, which is a circuit diagram of a backlight driving circuit for LEDs according to a prior art, and the light adjustment thereof is controlled by a pulse width modulated signal (PWM dimming signal). When the voltage of the PWM dimming signal has a constant DC high level, the MOS tubes, conducted with a constant current of the LEDs and connected in series with the LEDs in the IC, is always in the conducting state, the current of the LEDs is constant, then their brightness is the highest. When to dimmer, because the PWM dimming signal has a rectangular wave, the MOS tubes in the IC switches between on and off, the LED's current has also a rectangular wave, and because the average value of the LED's current declines, the brightness is reduced.
During the dimming, the PWM signals of every LED strings are same, that means the MOS tubes connected in series with the LEDs switch between on and off at the same time. In each working cycle, because of the large output power range, the current of the inductors may change frequently between the maximum and minimum values, which generates noise, impacts the components of the circuit, and results to a big wave of output voltage, so the circuit is unstable.